1. Field of the Invention
The present invention relates to a semiconductor pressure sensor and, more particularly, to an improved semiconductor pressure sensor which is capable of forming a diaphragm on the substrate thereof using a thin-film forming technique.
2. Description of the Prior Art
[Structure]
FIG. 8 shows the general structure of a conventional semiconductor pressure sensor. This semiconductor pressure sensor includes an N-type silicon substrate 10 and a pedestal 12. The substrate 10 is etched from the back side so as to form a diaphragm 14 about 20 to 50 .mu.m in thickness in the central portion of the substrate 10. The substrate 10 is adhered to the pedestal 12 at the thick-walled portion on the back side so as to form a reference pressure chamber 16 therebetween.
On the upper side of the diaphragm 14 of the N-type silicon substrate 10 strain gages 18 consisting of P-type resistance regions are formed by diffusion or ion implantation. On the upper surface of the substrate 10, insulating film 20 of silicon oxide or the like and electrodes 22 are formed.
According to the semiconductor pressure sensor having the above-described structure, the diaphragm 14 is deflected in proportion to the pressure being measured, and this deflection is detected as a change in resistance of the strain gages 18, whereby the pressure is measured.
In order to measure an absolute pressure by means of the semiconductor pressure sensor, the reference pressure chamber 16 provided between the substrate 10 and the pedestal 12 is made vacuum.
Thus, the diaphragm 14 is deflected in proportion to the absolute pressure applied to the surface thereof, and the deflection is electrically measured as a change in resistance of the strain gages 18.
When a differential pressure is measured by means of the semiconductor pressure sensor, a pressure introducing hole 24 which communicates with the reference pressure chamber 16 is provided in the pedestal 12, and the diaphragm 14 is deflected in correspondence with the difference in the pressure applied to the upper side and the underside of the diaphragm 14. Thus, a difference pressure is measured in a similar way to that of the absolute pressure type sensor.
In such a conventional type of sensor, the diaphragm 14 and the reference pressure chamber 16 are provided by means of backside-etching. When the substrate 10 is etched in this way, an anisotropic etching process using potassium hydroxide (KOH) water solution, etc. is widely adopted.
The reason for this is as follows. Since the etching speed on the plane (111) is very slow during etching, a (100) or (110) silicon substrate is used to form an etching mask of silicon nitride (Si.sub.3 N.sub.4) or the like on the under surface of the substrate 10, so that etching in the vertical direction proceeds in a tapered configuration having a regular angle .theta. of inclination. Thus, the diaphragm 14 and the reference pressure chamber 16 shown in FIG. 8 are formed.
The aforementioned conventional semiconductor pressure sensor, however, has several problems remaining to be solved which will be described in detail in the following, and effective countermeasures have been demanded.
(a) Since both surfaces of the silicon substrate 10 must be subjected to wafer processing, the processing steps become very complicated.
As described before, when the semiconductor pressure sensor is fabricated, it is necessary to form the strain gage 18s, the insulating film 20 and the electrodes 22 on the upper surface of the substrate 10, and to subject the under surface thereof to etching masking and anisotropic etching in order to form the diaphragm 14.
Since such wafer processing on both sides of the substrate 10 requires photoecthicn process for each step using a both-side alignment device and each processing step inevitably becomes complicated.
Even if such a both-side alignment device is used, an error is produced to a certain extent when positioning the strain gages 18 in alignment with the peripheral portion of the diaphragm 14, such error causing non-uniformity in the sensitibity of the sensor.
Generally, a torelance of about 5 .mu.m is allowed in double-sided alignment when a 300 .mu.m-thick silicon substrate is used. The magnitude of errors in alignment increases with increase in the thickness of the silicon substrate. Especially when a large-diameter silicon wafer is used, the characteristics of each sensor vary greatly, so that mass production of such sensors is very difficult.
(b) It is difficult to reduce the thickness of the diaphragm 14 of the conventional semiconductor pressure sensor.
In the conventional semiconductor pressure sensor, as is known, a diaphragm 14 of desired thickness is formed by adopting a method wherein etching is stopped at a time obtained by calculation on the basis of the etching speed in the direction of the depth of the silicon substrate 10.
However, this etching speed varies depending on the surface condition of a wafer or the number of wafers and, in addition, the thickness of the substrate 10 itself is not uniform even though it may fall within a predetermined tolerance range. As a result, it is inevitable that the thickness of the diaphragm 14 formed by such etching process is not sufficiently uniform even though it may fall within the predetermined tolerance range.
Generally, it is necessary to allow for an error of about 2 .mu.m when the diaphragm 14 of 20 to 50 .mu.m thick is formed.
The sensitivity of a semiconductor pressure sensor is inversely proportional to the square of the thickness of the diaphragm. Therefore, the sensitivity of the pressure sensor varies closely in accordance with any error in the thickness of the diaphragm.
Consequently, the thickness of the diaphragm 14 of the conventional semiconductor pressure sensor is at least about 5 .mu.m, which makes it impossible to obtain a sensor displaying high-sensibility.
(c) It is difficult to make the dimension of the diaphragm 14 small in a conventional semiconductor pressure sensor.
The dimension of the diaphragm 14 is determined by the dimension of the etching mask provided on the under surface of silicon substrate 10, the thickness of the silicon substrate 10, the depth of etching in the vertical direction during etching, and so forth.
When the diaphragm 14 is formed by anisotropic etching, etching proceeds inwardly from the periphery of the opening of the etching mask in a tapered configuration in accordance with the regular angle .theta. of inclination which is determined by the crystalline orientation, and the frustoconical reference pressure chamber 16 which is surrounded by the plane (111) is finally obtained.
The dimension of the diaphragm 14 formed as a result of such etching is determined by the dimension of the opening of the etching mask and the depth of etching in the vertical direction of the silicon substrate 10.
However, since it is necessary to allow for variation in the thickness of the silicon substrate 10 within a predetermined tolerance range at the beginning of etching, it is necessary to correct the depth of etching in the vertical direction of the substrate 10 by an amount equivalent to the difference in thickness by increasing or decreasing the depth of etching by that amount.
Consequently, it is inevitable that the depth of etching in the vertical direction of the substrate 10 varies within a predetermined tolerance range and, hence, the final dimension of the diaphragm 14 also varies within the predetermined tolerance range in correspondence with the variation in the depth of etching in the vertical direction.
If the variation of the thickness of the silicon substrate 10 is .DELTA.t, the variation of the dimension of the diaphragm is represented by 2.DELTA.t/tan .theta..
Accordingly, if it is assumed that, for example, the (100) plane silicon substrate 10 is used and a diaphragm of a rectangular shape having its sides disposed in the direction (110) is formed, the angle of inclination is about 55 degrees, and since it is necessary to allow about 10 .mu.m for the variation in the thickness of the diaphragm 14, the final dimension of the diaphragm 14 has a scattering of about 14 .mu.m.
As a result of this predetermined degree of variation of the diaphragm 14 formed by etching, the relative position of the periphery of the diaphragm 14 and the strain gage 18 varies. The amount of strain applied to the gages 18 is therefore varied, thereby producing variation in the sensitivity of the sensor itself.
In addition, since the degree of dimensional variation increases with the reduction in the dimension of the diaphragm 14 in the conventional semiconductor pressure sensor, the permissible dimension of the diaphragm 14 is at least 500 .mu.m in diameter or breadth. It is thus inconveniently impossible to make a sensor having a smaller diaphragm 14.
(d) It is necessary to adhere the silicon substrate 10 to the pedestal 12 in an airtight state to form the reference pressure chamber 16 in the conventional semiconductor pressure sensor.
When an absolute pressure is measured using the sensor, it is necessary to use a vacancy formed between the adhered substrate 10 and pedestal 12 as the reference pressure chamber 16 and to maintain the pressure within the reference pressure chamber 16 at a vacuum.
However, techniques for airtight adhesion of high efficiency such as, for example, anode bonding and glass bonding are necessary for adhering the substrate 10 to the pedestal 12. Furthermore, if there is the slightest leakage at the junction, the output characteristic of the pressure sensor varies over time.
In particular, in a sensor for measuring a pressure with high accuracy, the airtight adhesion technique employed is critical, which mitigates against the mass production of sensors.
The conventional semiconductor pressure sensor has the problems (a) to (d) described above. These problems make it difficult to enhance the measuring accuracy, to miniaturize a sensor, and to provide mass-produced and, hence, low-cost sensors due to the complicated processing steps.
As a result, in spite of the efficient performance of the conventional semiconductor pressure sensor, it has not yet been brought into general use, and an effective way of dealing with this drawback has become highly desirable.
Semiconductor pressure distribution detecting apparatus consisting of a plurality of semiconductor pressure sensors arrayed in matrix have been known and been widely used for dynamically detecting various pressure distributions, because they can detect dynamic change in pressure simultaneously at a plurality of points.
One of the proposals made with respect to such semiconductor pressure sensors is the pressure distribution detection apparatus disclosed in IEEE Trans. Electron Devices. Vol. Ed - 32, p. 1196, 1985, which is composed of a plurality of sensors integrally provided on a substrate.
FIG. 26 shows an example of this pressure distribution detecting apparatus. The detecting apparatus is composed of a silicon substrate 10, a glass pedestal 13, a frame body 15, and a plurality of electrostatic capacitance type semiconductor pressure sensors 200 arrayed on the silicon substrate 10 in a matrix of n rows.times.m vertical lines (hereinunder referred to simply as "lines").
Each of the semiconductor pressure sensors 200 includes the diaphragm 14 formed by removing the upper surface and the under side of the substrate 10 by etching, and the reference pressure chamber 16 formed by closely bonding the substrate 10 with the glass pedestal 13 at the thick-walled portion on the back side of the substrate 10. A first electrode 25 and a second electrode 27 are opposed within the reference pressure chamber 16.
The frame body 15 is adhered to the silicon substrate 10 at the thick-walled portion on the top side. A plurality of pressure introducing holes 17 for applying pressure to the diaphragm 14 of each sensor 200 which is arrayed in a matrix are formed on the frame body 15. The top side of the frame body 15 is covered with a coating 19.
When a pressure is applied from the top side of the frame body 15 of the apparatus having the above structure, the pressure is applied to the diaphragm 14 of each sensor 200 through the pressure introducing hole 17 and is detected as a change in electrostatic capacity between the first electrode 25 and the second electrode 27.
Thus, since a plurality of semiconductor pressure sensors 200 are integrally provided on the same silicon substrate 10, it is possible to array each sensor 200 in a dense matrix, so that the resolution for two-dimensional detection of distribution of the applied pressure is as good as 2 mm.
This pressure distribution detecting apparatus, however, has the following problems as well as the aforementioned problems (a) to (d), and an effective way of overcoming them countermeasure has been demanded.
(e) In this conventional apparatus, the silicon substrate 10 on which the first electrode 25 is attached to the under surface of each of the diaphragms 14 formed in a matrix and the glass pedestal 13 on which the second electrodes 27 are provided in a matrix are closely bonded. Thus, the capacitor which is composed of the first electrodes 25 and the second electrodes 27 detects a pressure.
If there is the least error in positioning the silicon substrate 10 and the glass pedestal 13, the electrostatic capacitance between the electrodes 25 and 27 which are provided in each sensor 200 takes a different value from each other, so that it is impossible to measure the pressure distribution accurately.
(f) Since the electrostatic capacitance type semiconductor pressure sensor 200 is used in this apparatus, floating capacity sometimes causes measuring errors.
In order to reduce the level of any error due to floating capacity, it is necessary to enlarge the area of the first electrode 25 and the second electrode 27 of each sensor. The area per sensor is therefore increased and the resolution for detecting pressure distribution is inevitably lowered.
Electrostatic shielding may be provided in order to reduce the level of any error due to efloating capacity, but the entire structure of the apparatus will then become very complicated.
(g) Since an electrostatic type sensor 200 is used, an output signal changes only slightly in correspondence with a change in electrostatic capacitance. A means for positively detecting such a slightly changed output signal is required, which makes the entire apparatus complicated and expensive.
As described above, the conventional electrostatic capacitance type pressure distribution apparatus has the above-described problems (a) to (g), which are obstacles to enhancement of measuring accuracy and miniaturization of the apparatus. In addition, since the manufacturing method is complicated, mass production and, hence, reduction in cost are unrealizable.
At present, a pressure distribution detecting apparatus having highly-accurate two-dimensional resolution is required as, for example, a contact pressure sensor or tactile sensor for, for example, a precision work robot, or for other purposes. Therefore measures for overcoming these problems have been strongly demanded.